One method of controlling the advance of the air filter curtain in an automatic roll-type air filter is to sense the variations in the static pressure on opposite sides of the filter curtain, with a high pressure differential indicating a heavily dust laden media thereby requiring advancement of clean media into the filtering zone. Another known method of controlling the advance of air filter media into the air stream is to advance increments of filter curtain at predetermined intervals with the predetermination of the intervals being based upon an estimated rate at which the filter curtain will capture contaminants and dust from the air stream. When employing the latter method, it becomes apparent that if the estimate of contaminant capture is reasonably accurate, the use efficiency of the filter media will generally be high. However, if the dust concentration in the air varies over a wide range, this method is very inefficient and very uneconomical. Other known methods of advancing a filter media curtain include combinations employing both a static pressure differential variation detector and a time measuring apparatus in an attempt to gain the best of the two above-mentioned methods.
The present invention, however, is concerned with still another method of controlling the advance of a filter curtain in roll-type air filters. This method features the underlying concept that variations in the light transmitting character of a filter curtain, which passes light, may be used as the basis upon which to make a determination of the need for advancing additional clean filter curtain into the air stream. The principal underlying this concept is that the light transmitting character of the filter curtain will vary within limits in accordance with the quantity of contaminants or dirt caught by the filter curtain. More specifically, the principal is that the greater the amount of dirt captured by the filter curtain, the greater the reduction in light transmission through the curtain and, conversely, the less dirt captured, the less reduction in light transmission. Thus, the present invention provides a straightforward, inexpensive, readily constructed and easily maintained control circuit for automatic roll-type air filters which provides a photocell for detecting the variations in the light transmitting character of the filter curtain, means for preselecting a set-point at which the filter media curtain will advance in response to dust loading, and fail-safe means for preventing the advancement of the filter media curtain when clean filter media is present in the filtering zone of the roll-type air filter.